Bok choy or pak choi (Brassica rapa subsp. chinensis) is a type of Chinese cabbage. Chinensis varieties do not form heads; instead, they have smooth, dark green leaf blades forming a cluster reminiscent of mustard or celery. Chinensis varieties are popular in southern China and Southeast Asia. Being winter-hardy, they are increasingly grown in Northern Europe. This group was originally classified as its own species under the name B. chinensis by Linnaeus.[citation needed]

Other than the ambiguous term "Chinese cabbage," the most widely used name in North America for the chinensis variety is bok choy (from Cantonese, literally "white vegetable"; also spelled pak choi, bok choi, and pak choy). In the UK, Australia, South Africa, and other Commonwealth Nations, the term pak choi is used. Less commonly, the descriptive English names Chinese chard, Chinese mustard, celery mustard, and spoon cabbage are also employed.

In Australia, the New South Wales Department of Primary Industries has redefined many of these names to refer to specific cultivars. In addition, they have introduced the word buk choy to refer to a specific kind of cabbage distinct from pak choy.[1][2]

In China, three terms are commonly used for this vegetable: the majority of Chinese (about 955 million) speak Mandarin, and for them the term of a northern variant of the vegetable is 油菜 yóu cài (literally "oil vegetable"), since most of the cooking oil in China is extracted from the seed of this plant[dubious– discuss]; Shanghainese speakers (about 90 million in eastern China) use the term 青菜 qīng cài (literally "blue-green vegetable"); although the term 白菜 is pronounced "baak choi" in Cantonese, the same characters are pronounced "bái cài" by Mandarin speakers and used as the name for napa cabbage which they call "Chinese cabbage" when speaking English.

Bok choy contains glucosinolates. These compounds have been reported to prevent cancer in small doses, but, like many substances, can be toxic to humans in large doses, particularly to people who are already seriously ill. In 2009, an elderly diabetic woman who had been consuming 1 to 1.5 kg of raw bok choy per day, in an attempt to treat her diabetes, developed hypothyroidism, for reasons relating to her diabetes, resulting in myxedema coma.[5]